Ground Proximity Warnings

FOR A PILOT FLYING AT 15,000 FEET, nothing can spoil the day faster than a 16,000-foot mountain suddenly looming ahead.

Whenever an airplane is functioning properly but nonetheless slams into terrain, either because it is off course or because the pilot has lost track of his position, the technical term is “controlled flight into terrain”—CFIT, pronounced “SEE-fit.” Around the world, about four flights succumb to CFIT each year. Investigators recently found CFIT to be a factor when a Fokker F-28 slammed into a cloud-covered mountaintop in Peru last January 9, killing all 47 people aboard.

The Federal Aviation Administration attempted to reduce the incidence of CFIT by mandating in 1974 that aircraft operate with Ground Proximity Warning Systems (GPWS). But the systems had limitations. GPWS relied on the airplane’s radio altimeter, which determines the aircraft’s altitude by bouncing a radar signal off the ground and measuring the duration of the signal’s round trip. “The sensor was looking straight down,” says Greg Francois of Honeywell Aerospace. “If you were going into very steep terrain, then you got a very short warning”—10 to 15 seconds or less. And GPWS cockpit displays were crude; their sole visual warnings were lights. Even with GPWS, the pilot of that F-28 still flew his airplane into a mountain.

Today the FAA is banking on an improved technology: the Terrain Awareness and Warning System. The agenccy requires that in the United States, all new airliners (and by 2005, most older ones) be equipped TAWS. The new technology has the ability to look notjust down but also ahead, so that the crew can get a longer warning of possible CFIT: 30 to 120 seconds. And for visual warnings, TAWS gives detailed pictures of significant terrain.

TAWS uses the Global Positioning System to determine the aircraft’s position, ground speed, and ground track. That data, along with the aircraft’s altitude, are fed into a computer in the airplane. The computer has a database on the world’s natural terrain, man-made obstacles, and runways.

When the airplane’s position and flight path are superimposed on the database of the relevant area, the results are rendered as detailed visual representations of significant terrain and obstacles around the craft. The images appear either on a dedicated monitor or on the aircraft’s weather radar screen (in which case the pilot can toggle between weather and terrain displays).

Some manufacturers’ systems go farther, analyzing the flight plan in the aircraft’s flight management system computer and plotting potential hazards along the entire route.

Today’s TAWS monitors show terrain below the aircraft’s altitude as green or black, terrain near the aircraft’s altitude as yellow, and mountains or other terrain well above the aircraft’s altitude as red. When the aircraft is descending, the colors represent distances above or below its projected glideslope. When the aircraft is a minute or so from flying into the ground, the TAWS gives both visual and aural warnings.

TAWS uses the same warning modes that were used in the older GPWS:

Mode 1 warns of an excessive descent rate during landing or whenever the aircraft is close to the ground. The initial warning is the phrase “Sink rate”; if the problem isn’t corrected, a “Pull up” warning is issued.

If the aircraft is flying into the slope of a mountain or across other steep features and the ground clearance is rapidly decreasing, mode 2 issues an aural “Terrain” warning, followed by “Pull up.”

Mode 3 helps pilots maintain a positive climb rate after takeoff; once the aircraft reaches 1,000 feet, the system sounds a “Don’t sink” warning if altitude begins to descend.

Mode 4 warns if landing gear or flaps are not properly configured for landing, and if the terrain clearance is inadequate during final descent.

When the crew is making an instrument landing system—ILS—approach, mode 5 announces “Glide slope” if the aircraft has deviated below a safe flight path to the runway.

Another operating mode calls out altitudes during descent, and the last warns of significant wind shear.

Today, 18,000 commercial, military, business, and general aviation aircraft have TAWS. Jim Burin, director of technical programs at the Flight Safety Foundation, says the F-28 that crashed in Peru and two other aircraft victimized by CFIT this year—a 737 in Brazil and an RJ100 in Turkey—all had the older GPWS systems, not the newer TAWS.

As TAWS comes to predominate in coming years, aircraft may have an easier time staying in the skies—and away from unexpected mountain peaks.